The invention relates to ascertaining the vertical and lateral charge carrier distribution or doping profiles in semiconductor samples, which is generally known as profiling.
For the production of semiconductor components, it is important to know the charge carrier distribution and the variations of dopant concentrations in the semiconductor material. Not only the courses in terms of depth but also the lateral resolution capabilities are of interest. Various methods and equipment have become known for this purpose. Classical profiling methods include SIMS (Secondary Ion Mass Spectrometry), SR (Spreading Resistance), SH (Stripping Hall) and CVM (Capacitance Voltage Method). With the exception of the SR method, the methods have the disadvantage that their lateral resolution is in the millimeter range, which is very coarse. The SR method, in which the measurement is done via a scanning cantilever, has the disadvantage that the contact between the scanning cantilever and the semiconductor to be tested is highly dependent on the material and on the surface property. Maximum mastery of the SR method is achieved only in the testing of silicon samples.
Microscopy methods that work with fine cantilevers are also known. In particular, these are STM (Scanning Tunneling Microscopy), AFM (Atomic Force Microscopy) and SCM (Scanning Capacitor Miscroscopy). In these methods, if the samples are suitably prepared, both lateral and vertical resolutions of a few tenths of a nanometer up to a few tens of nanometers are possible. However, these methods have the disadvantages discussed below.
In STM, scanning is done in a grid pattern over the sample using a fine tip, and the local current/voltage characteristic curve is picked up. If an oblique grind or an edge is being examined, then the work function of the electrons is dependent on the local energy band distortion at the surface, which in turn affects the current/voltage characteristic. The greatest source of error in this method is the absence of direct information about the actual distance between the tip and the sample, so that temperature fluctuations and irregularities can have considerable influence on the current/voltage characteristic curves.
In an AFM scanning arrangement, a scanning wand or cantilever has a scanning tip that in the position of repose is spaced a predetermined distance from the sample. For measurement, the scanning cantilever is set into oscillation, at one of its possible resonant frequencies. The motion of the tip is detected with the aid of a laser that is focused on the reflecting scanning cantilever. Shifting of the resonant frequency as a result of force gradients between the tip and the sample causes variations in the amplitude of the oscillation, which are utilized in an optical feedback loop to keep the tip-to-sample spacing constant.
To use the AFM arrangement as a profiler, it is known (from M. P. Boyle et al, Advances in Dopant Profiling by Atomic Force Microscopy; Proceedings of the First International Workshop on the Measurement and Characterization of Ultra-Shallow Doping Profiles in Semiconductors, 1991, and J. Vac. Sci. Technol. B, Vol. 9, 1991, No. 2, 703-706) to apply a low direct voltage of from 1 to 2 V to the tip; superimposed on this is an alternating voltage of a few hundred millivolts, in order to set the tip into mechanical oscillation at another resonant frequency of the scanning cantilever. (The disclosure of the Boyle et al article is incorporated herein fully by reference.) As a result, the surface space charge zone, which is normally always present, is modulated about its state of equilibrium, and a differential capacitance is thus generated.
Measuring this variable makes it possible to determine only the near-surface doping for maximally homogeneously doped semiconductors. Another disadvantage of the previously known achieved embodiment of this method is that until now there have been no suitable evaluation algorithms. The simplified assumptions of a plate capacitor cause considerable error.